Modified optimum pitch propeller

ABSTRACT

An asymmetric set of pre-swirl vanes (stators) and a specially matched propeller for use on an inclined shaft. The propeller is designed by considering the mutual interaction of the propeller on the vanes and the vanes on the propeller. The propulsor unit provides the following: 
     1. increased propulsion efficiency due to the reduced rotational (swirl) and axial kinetic energy losses in the propulsor&#39;s slipstream; 
     2. reduction or elimination of propeller cavitation; 
     3. reduction or elimination of unsteady propulsor forces as well as propulsor-induced hull vibrations. 
     A unique feature of the present invention is that a prior art flat faced commercially available propeller can be modified to match the vane flow field for optimum propulsor performance. The use of commercially available propellers reduces the installation or hardware cost significantly and allows the propeller to be repaired easily if damaged. 
     Another unique feature is that the vanes operate well with an unmodified commercially available prior art flat faced, optimum constant pitch propeller, and that the propeller as modified for use with the vanes also performs exceptionally well without the vanes. The modified propeller without vanes in fact outperformed the prior art flat faced optimum constant pitch propeller used on the 41 foot test craft.

STATEMENT OF GOVERNMENT INTEREST

The present invention may be used by or on behalf of the Government ofthe United States without the payment of any royalties thereon ortherefor.

This application is a division of application Ser. No. 504,528, filedMar. 14, 1990, now abandoned, which is a division of Ser. No. 163,878filed Mar. 3, 1988 now U.S. Pat. No. 4,932,908.

BACKGROUND

The prior art has recognized that improvements in efficiency could beobtained from properly directing the flow of water into a propeller.Previous systems had vane (stator)-propeller combinations in which thevanes were located either forward of the propeller (pre-swirl), or aftof the propeller (post-swirl). These systems have one or more of thefollowing in common:

1. the vanes are mounted axisymmetrically, and are designed for the casein which the flow is perpendicular to the propeller disc.

2. The vanes are designed to work in the viscous boundary layer of theship; in this respect the vanes are operating as a flow directing deviceonly.

3. A specially designed propeller (not a modified commercially availableoff-the-shelf propeller) is used with either of the above vanes.

Recently, however, it has been recognized that in some cases the flowdirecting means should not be symmetrical since the flow into thepropeller is not symmetrical. See, for example, Japanese patentapplication number 56-162006 (found in U.S. class 440 subclass 66) whichshows a ship having a single propeller and which has a set offlow-directing vanes on only one side of its stern, the purpose of whichis to create a wake stream flowing in the opposite direction to theturning direction of the propeller. The propeller shaft of the ship ishorizontal; the flow distortion that the vanes are intended to overcomeis caused by the boundary layer close to the hull.

Japanese patent application number 58-77998 (also found in U.S. class440 subclass 66) shows a ship having dual propellers mounted on struts,one on each side of the stern. In this application the struts, which areasymmetrically arranged around the propeller disc, are contoured toprovide water flow to the propeller with a rotary component opposite tothe rotation of the screw propeller. However, in this application thepropeller shafts are also horizontal with the flow distortion beingcaused by the shape of the stern of the vessel.

What the prior art has failed to recognize is that the flow into apropeller that is mounted on an inclined shaft is oblique and causes aonce-per-revolution variation in propeller blade section angle ofattack. The prior art also failed to recognize that the propeller on anoutboard motor is also inclined to the water flow when the boat ismoving.

OBJECTS OF THE INVENTION

Accordingly, it is an object of the present invention to provide a meansof increasing the efficiency of a propeller mounted on an inclinedshaft.

It is a further object of the present invention to provide suchincreased efficiency on a vessel having either a single propeller ormultiple propellers.

It is a further object of the present invention to provide suchincreased efficiency regardless of the configuration of the vessel'sbottom.

It is a further object of the present invention to provide suchincreased efficiency without the addition of moving parts to thepropulsion system.

It is a further object of the present invention to provide suchincreased efficiency in an outboard motor.

It is a further object of the present invention to provide a means ofincreasing propeller loading while minimizing cavitation problems.

It is a further object of the present invention to provide suchincreased efficiency in an airplane propeller.

It is a further object of the present invention to provide a propellerguard for a vessel that does not result in a net loss in propulsionefficiency for the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the reductions in propeller RPM for any given speed afterthe vane-propeller system of the present invention is installed.

FIG. 2 shows how a change in engine RPM causes reductions in fuelconsumption rates on a diesel engine fuel map.

FIG. 3A and 3B show the forces acting perpendicular to the propeller.

FIG. 4 is a side view of the 41 foot boat that was used in thedevelopment of the present invention showing the propeller on itsinclined shaft.

FIGS. 5 and 5A show the inflow velocities seen by the propeller disc.

FIGS. 6a, 6b, and 6c shows the variation in blade section angle ofattack as the propeller makes a complete revolution.

FIGS. 7A and 7B show the forces asociated with asymmetric vanes on aninclined shaft.

FIGS. 8a, 8b, and 8c shows how the vane-propeller system of the presentinvention reduces or eliminates variations in the blade advance angle β.

FIG. 9 is a side view of a vane as designed for the 41 foot boat.

FIG. 10 shows the maximum camber of the vanes.

FIG. 11 shows an end view of the vanes.

FIG. 12 is a view from underneath the boat looking toward the stern.

FIG. 13 shows the application of the principles of the present inventionto an airplane propeller.

FIG. 14 shows the coordinates for the trim tab or flap.

FIG. 15 shows the rectangular section offsets for port vanes 1 and 2 ofthe example.

FIG. 16 shows the rectangular section offsets for port vanes 3 and 4 ofthe example.

SUMMARY

Briefly, the present invention comprises a set of asymmetric pre-swirlvanes and a matched propeller, the vanes being located asymmetricallyaround the propeller disc. Most of the vanes are located on the side ofthe disk where the propeller is on the upward part of its rotation.Increased propulsive efficiency results from the following:

1) reduced axial and rotational kinetic energy losses in the slipstreamof the propulsor;

2) reduced viscous friction losses on the propeller blades;

3) a more optimum loading on the propeller;

4) a reduction in engine RPM which allows a marine diesel to operate ina more efficient area of its fuel map for a given craft speed;

5) reduction or elimination of vessel drag associated with forces whichresult from having a propeller mounted on an inclined shaft.

The asymmetric pre-swirl vanes and matched propeller system of thepresent invention provide the following, either singly or incombination:

1) increased propulsive efficiency;

2) reduction in propulsor-induced hull or machinery vibrations;

3) improved directional stability on single propeller vessels;

4) counteraction of torque on single propeller vessels;

5) reductions in propeller cavitation and cavitation damage;

6) reduction in propeller diameter without losing original efficiency;

7) protection from injury by the propeller to persons in the waterwithout a net loss in propulsive efficiency due to drag of theprotective device.

Although the vanes and modified propeller of the present invention weredesigned as a unit, it was unexpectedly found that each could operateindependently of the other. That is, the vanes can operate with anunmodified prior art commercially available flat faced optimum constantpitch propeller to produce increased efficiency over the whole speedrange of the craft. Further, such a propeller, when modified inaccordance with the present invention, can produce efficiency gains whenoperating without the vane of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 4 shows boat 10 for which the asymmetric vanes and matchedpropeller were developed. Boat 10 is a standard Coast Guard patrol boatof 41 feet overall length having twin propellers. As can be seen,propeller 12 is mounted on inclined shaft 14 which makes an angle ofabout 14 degrees with respect to the boat bottom (or "buttock lines").Because of the angle of shaft 14 with respect to the bottom of boat 10,propeller 12 makes a corresponding angle of 14 degrees with the inflowof water, since the water flow is approximately parallel to the bottomof the boat.

FIG. 5 shows a side view of the magnitudes and directions of water flowcomponents in vector form "seen" by a propeller that does not have thevanes of the present invention. Velocity vector V1 represents the actualwater flow parallel to the bottom of the boat. Velocity vector V2represents the component of water flow parallel to propeller shaft 14.Since a given water molecule which starts at point 18 has to reach point20 at the same time whether it follows path V1 or V2, this means thatthere must be a velocity component such as is represented by velocityvector V3 for the water that flows parallel to shaft 14. FIG. 5a showsthe way that propeller 12 "sees" velocity vector V3; that is, the waterapproaching propeller 12 appears to have a uniform upward velocity equalto the magnitude of velocity vector V3. It should be noted that thewater approaching propeller 12 is undisturbed by the stern of the vesselor any protuberances on the underside of the vessel. In this example thepropeller is assumed to have a counterclockwise rotation; therefore, forthe first half of the propeller's rotation (from 0 degrees to 180degrees in FIG. 5a) this velocity is counter to the propeller'srotation, while it is in the direction of the propeller's rotation forthe second half of the propeller's rotation. A propeller produces lessthrust on the side where it is moving in the same direction as the waterflow into it (i.e. the second half of the propeller's rotation in FIG.5a) than on the side where it is moving in the opposite direction to thewater flow into it (i.e. the first half of the propeller's rotation inFIG. 5a). This is because velocity vector V3 in FIG. 5 causes the bladesection angle of attack to change periodically as the blade makes onerevolution, as shown in FIG. 6. In this figure, a single blade sectionis shown at the 0, 90, 180, and 270 degree positions in the propellerdisc. Velocity vector V2 is the same as in FIG. 5. The tangentialvelocity Vt of a blade section at radius r and moving at n revolutionsper second is Vt=2πrn. As can be seen, velocity V3 is added to andsubtracted from Vt at 90 degrees and 270 degrees, respectively. Thiscauses the blade advance, β, to change, which results in variations inthe section angle of attack, which in turn causes changes in the loadingof the propeller blade. Therefore, the loading of the propeller blade isgreatest at 90 degrees and least at 270 degrees. For other locations onthe propeller disc β is given by the following formula:

    β=tan.sup.-1 [V2/(2πrn+V2 tan φ sin φ)]    (1)

where

θ=angular position on propeller disc (see FIG. 4)

r=vector position of blade section

φ=angle between bottom of boat and shaft (see FIG. 5).

The variation in blade section angle of attack at any propeller radius,hence the loading, is approximately the difference between the advanceangle, β, given in equation (1), and the pitch angle of the blade atthat radius. This once-per-revolution variation in angle of attack givesrise to unsteady axial forces along the propeller shaft, and steady aswell as unsteady forces which are perpendicular to the inclined shaft(see FIG. 3). When this perpendicular force is resolved into itshorizontal and vertical components it can be seen that the horizontalcomponent is in reality a drag force Dp acting on the craft. As shownbelow, the asymmetric pre-swirl vanes of the present invention minimizeor eliminate this drag.

The drag of the vanes resolved parallel to craft motion, Dv, is composedof three components: 1) viscous drag; 2) induced drag; and 3) drag dueto inclination of the flow relative to craft advance (see FIG. 7A). Theviscous drag is composed of friction and eddy formation losses. Induceddrag results from the trailing vortex system of the vanes. Finally, theasymmetry of the vanes results in a force which acts at an oblique angleto the shaft and at a right angle to the inflow and has a component offorce directed aft. When all three drag components are resolved parallelto the direction of craft motion a net drag, Dv, results.

The rotational velocity field, V(θ)_(swirl), at the propeller disc,caused by the vanes, is shown in FIGS. 8a, 8b, and 8c. The rotational orswirl velocities induced by the vanes have reduced or eliminated thecyclic variations in the advance angle, β, and, therefore, haveminimized or eliminated the perpendicular shaft force and its dragcomponent Dp. The decrease in perpendicular force generated by thepropeller is offset, either totally or partially, by the increase in thecomponent of vane force which is perpendicular to the propeller shaft.

The reduction or elimination of variations in the advance angle alsoreduces the probability of face cavitation. In addition, the vanes tendto decrease the load on the more heavily loaded portion of the propellerdisc, as will be discussed later. This also reduces the amount orlikelihood of cavitation on the blade backs.

For a given craft speed a reduction in propeller RPM occurs whencompared to the same craft without the vane-propeller combination,FIG. 1. Since frictional energy losses on the propeller blades (due tothe viscosity of the water) are proportional to the square of thepropeller RPM, a significant increase in propeller efficiency willoccur. In addition, for the typical diesel engine a reduction in RPM fora given craft speed results in a reduction in the fuel consumption rateof the engine. FIG. 2 shows this phenomenon which occurred on the testcraft. The savings in fuel from this effect amounted to approximately 2%of the total fuel savings. This is in addition to the savings realizedfrom a reduction in shaft horsepower required for a given speed.

For single propeller vessels with inclined shafts but without vanes aforce which has steady and unsteady components, see FIGS. 3A and 3B,acts perpendicular to the shaft. The horizontal component of the steadyforce acts to turn the vessel. This turning has to be counteracted byuse of the rudder and a loss of energy occurs due to the increase indrag caused by the rudder deflection. When properly matched vanes areadded, the side force from the propeller is offset by the side forcefrom the vanes shown in FIG. 7B. The side force from the vanes alsopartially or completely offsets the torque of the propeller in a singlepropeller vessel.

Due to inclination of the propeller shaft one side of the propeller disc(without vanes present), the side where the propeller is on the upwardpart of its rotation, is lightly loaded while the other side is heavilyloaded. As a single propulsor unit the asymmetric pre-swirlvane-propeller combination can be designed for a more uniform loading ofthe propeller disc. This results in a further reduction in axial kineticenergy losses in the propulsor slipstream. By reducing the load on theheavily loaded side of the propeller disc, blade back cavitation isreduced or eliminated. Further, on the lightly loaded side of thepropeller disc, face cavitation may occur. By loading up this side ofthe disc with the vanes, blade face cavitation will be eliminated.

Due to the uneven loading on prior art propeller discs without vanes,vibrations resulting from unsteady forces which are perpendicular to theshaft (inclined shafts only) and parallel to the shaft occur. Thesevibrations are transmitted up the shaft to the reduction gear or engineand are also transmitted through the shaft bearing directly to the hull.By using a properly designed asymmetric pre-swirl vane and matchedpropeller the unsteady forces caused by the propeller can be minimizedor eliminated. This results in reduced hull-borne vibrations, a quietervessel, increased propeller shaft bearing life, and reduced maintenanceon reduction gears and engines.

Pre-swirl vanes also act as a guard for the propeller. Any loss inefficiency caused by the vanes is offset by the increased efficiency ofa properly designed propulsor unit. The guard can prevent debris fromhitting the propeller while in operation or prevent injury to humanswhile boarding or swimming in the vicinity of a turning propeller.

A propeller on an inclined shaft produces a force which is perpendicularto the shaft. A component of this force acts as a drag on the craft inthe direction of craft travel as shown in FIG. 3A. The use of theasymmetric pre-swirl vane and matched propeller minimizes or eliminatesthis drag caused by the propeller.

When properly designed the asymmetric pre-swirl vane and matchedpropeller combination can have a smaller diameter propeller compared tothe prior art optimum propeller diameter without vanes. This allows moreclearance between hull and propeller, and reduces the vessel's draft.

FIG. 9 is a view looking down from the top of the propeller bearingstrut. The trim tab shown attached to the strut acts similarly to a flapon an airplane wing. This tab or flap generates a circulation around thebearing strut, creating a horizontal force on the strut. Therefore, thestrut and trim tab generate tangential velocities or swirl opposite tothe rotation of the propeller, and are considered an integral part ofthe propulsor design. FIG. 9 shows that each vane is shark-fin inoutline; this is for the purpose of shedding debris from the vanes andpropellers, since debris is a common problem in the waters where theseboats operate. The chord length of the vanes at r/R (R is the tip radiusof the vane, r is local radius)=0.25 is 10.3 inches and is linearlytapered to 2.13 inches at r/R=0.96. The vane offsets, including upper(suction side) and lower (pressure side) vane thickness coordinates,pitch distribution, and chord lengths at four non-dimensional radii, aregiven in FIG. 15 for vanes 1 and 2 (see FIG. 5a) and in FIG. 16 forvanes 3 and 4. These offsets are for the vanes for the port propeller;the vanes for the starboard propeller are mirror images of these vanes,assuming that the propellers are counterrotating. FIG. 14 gives theoffsets for the strut trim tab or flap.

The camber distribution of the vanes is approximately that of an NACA 65airfoil mean line, and the spanwise distribution of maximum camber isshown in FIG. 10. Note that vanes 1 and 2 have a slight reduction inpitch near their tips while vanes 3 and 4 do not. This reduction inpitch unloads the vane tips on these two highly loaded vanes. Thenose-tail line of the root sections of vanes 1 and 2 is set at a 2degree angle relative to the centerline of the shaft and that of vanes 3and 4 is set at 5 degrees relative to the centerline of the shaft. Thesection angle of attack of a vane section relative to the inflow at anyradius is determined approximately by the following formula:

    α=ε-φ sin θ                        (2)

where:

α=section angle of attack

ε=angle of vane root section with respect to the shaft centerline

φ=angle of shaft with respect to the craft's bottom or buttock lines

θ=angular position of the vane with respect to the bearing strut; 0degrees is at the strut, and 90 degrees is at the 9 o'clock positionlooking forward from behind the propeller.

When the section camber is included with the section angle of attackgiven by equation (2) to determine loading, it can be seen that eachvane is loaded differently. Vanes 1 and 2 have the greatest loading,with a slightly reduced amount on vane 3. The lightest loadings occur onthe strut trim tab and on blade 4. This results in very large, localtangential velocities being induced at and downstream of the propellerby the vanes.

This asymmetric tangential velocity field is what is responsible for thestated improvements in propulsive efficiency. The rotational velocityfield or tangential velocity field induced by the propeller in itsslipstream is partially cancelled out by the counter-rotation of fluidinduced by the vanes. This produces a portion of the stated energysavings. A naturally occurring partial cancellation of the swirlvelocities induced by the propeller in its slipstream results from thesuperposition of vector V3 (FIG. 5) onto the side of the slipstream notcovered by the vanes.

The distance from the vane hub to the vane tip is 10 inches, which isapproximately 77 percent of the propeller radius (the propeller radiusis 13 inches). The vanes can extend from about 75% to about 100% of thepropeller blade radius; the actual length for a given application willdepend on the operating conditions for that application. As statedearlier, the vanes are mounted on the shaft bearing housing immediatelyahead of the propeller.

FIG. 11, which is a view along the centerline of the propeller shaft,shows the vanes skewed counterclockwise with respect to a radial lineintersecting the mid-point of the vane root. This skew was necessary dueto the the method of vane construction. On future designs this skew mayor may not be necesary, depending on construction methods. FIG. 12 is aview of the vanes from underneath the test craft looking toward thestern and shows the vane-propeller combination as seen by the incomingwater flow. Vanes 16 are primarily on that part of the propeller discwhere the blades are on the upward port of their rotation, since this isthe part of the disc where the blades are lightly loaded due to theinclination of the propeller shaft.

The propeller initially used on the test craft was an off-the-shelf,flat faced, optimum constant pitch propeller manufactured by ColumbianBronze Corp. An identical propeller, except of lower original pitch, waslater mechanically repitched to match the perturbation velocity fieldgenerated by the vanes. The characteristics of this re-pitched propellerare shown in Tables I and II. The ability to use a modified commerciallyavailable propeller with the vanes is important since it considerablyreduces the installation cost of the asymmetric preswirl vanes andmatched propeller set. Physical constraints prevent an exact match ofthe propeller to the vanes when mechanically repitching; however, wheremaximum performance or efficiency is the primary consideration aspecially designed and manufactured propeller can be used. Initial cost,however, will increase sharply.

                  TABLE I                                                         ______________________________________                                        Modified Propeller Specifications                                             ______________________________________                                        Diameter         26.0 inches                                                  Pitch and Chord lengths                                                                        See Table V                                                  Hub length       6.0 inches                                                   Blade thickness  Same as the 26" × 28"                                  distribution     TETRADYNE* series propellers                                 Blade skew angle Same as the 26" × 28"                                  distribution     TETRADYNE* series propellers                                 Blade rake       Same as the 26" × 28"                                                   TETRADYNE* series propellers                                 Number of propeller blades                                                                     Four                                                         Shaft diameter   2.0 inches                                                   Shaft taper      Standard SAE J755 taper                                      Material         Ni-BRAL, ABS grade 4                                         ______________________________________                                         *Manufactured by Columbian Bronze Corp.                                  

                  TABLE II                                                        ______________________________________                                        Pitch and Chord Distributions of Modified Propeller                           r/R         Pitch (inches)                                                                           Chord (inches)                                         ______________________________________                                        0.30        24.80      5.99                                                   0.40        25.45      7.54                                                   0.5.        26.02      8.89                                                   0.60        26.57      9.88                                                   0.70        27.30      10.39                                                  0.80        28.29      9.96                                                   0.90        29.49      7.87                                                   0.95        30.52      5.65                                                   ______________________________________                                    

Two methods of designing the vanes and propeller system presently exist.The first method is by modified momentum theory and the second is bylifting line and lifting surface theory. The latter is the preferredmethod since local velocities and pressures can be predicted. Using thismethod, perturbation velocities from the vanes are calculated at anddownstream of the propeller disc. In turn, perturbation velocities whicharise from the propeller are calculated at the vane location. Thisprocess is repeated until convergence occurs. The final calculatedperturbation velocities along with the design requirements dictate vaneand propeller geometry.

As with all methods used for propulsor design, model or full scale testsare required. Generally, small changes in propulsor geometry will berequired after the first series of tests. Therefore, the vane andpropeller system of the present invention is designed to be as close tothe optimum geometry as the present state of the art permits, followedby model or full scale tests, and possibly small geometry changes.

It was also discovered that the vanes and the modified propellerfunctioned well independently of each other. When the asymmetricpre-swirl vanes were located ahead of the stock or original propeller(26 inch diameter, 28 inch optimum constant pitch "Tetradyne" propeller)on the test craft, reductions in shaft horsepower of up to 6 percentwere recorded. More significantly, reductions in fuel consumption of upto 15 percent were recorded at speeds of 10 knots; however, as speedincreased to 23 knots the fuel savings vanished. Other benefits similarto those resulting from the use of the vanes and matched propeller werealso realized, but to a significantly lesser degree. The high reductionin fuel consumption at lower speeds, using the original propeller and anadd-on set of asymmetric vanes, could be very important on craft whichoperate at low speeds for long periods of time, such as work boats andtrawlers.

The modified stock propeller used with the vanes also performedextremely well without the vanes on the inclined shaft. The use of thispropeller alone resulted in significant reductions in shaft horsepowerand fuel consumption over the speed range of the craft. It is believedthat the loading up of the highly pitched blade tips, and the unloadingof the root sections, are responsible for the performance gains. Thecurrent state of the art in propeller design, where no viscous wake isassumed to exist, dictates that a constant pitch propeller be used inthis application for maximum efficiency. After reviewing test data forthis propeller alone, it is believed that two phenomena occurred: 1)Unsteady forces related to the once-per-revolution variation in bladesection angle of attack, which is greatest at the inner radii of thepropeller, are reduced when the pitch--hence load--at the inner radii isreduced, thereby causing a reduction in energy losses related to theseunsteady forces. 2) At the outer propeller radii, theonce-per-revolution variation in blade section angle of attack becomesminimal; therefore, loading up the blade tips causes thrust to bedelivered in a more uniform manner over the annulus swept out by theouter portion of the tips, and this in turn minimizes axial androtational kinetic energy losses in the slipstream of the propeller.

The performance of the modified propeller on an inclined shaft cannot bepredicted with current analytical or empirical means. However, it isbelieved that if one selects a constant pitch, flat faced propeller foroptimum performance, based on non-inclined shaft analysis, theperformance of this propeller can be increased using the followingguidelines: 1) select a blade root pitch which is 10 percent less thanthe optimum constant pitch propeller. 2) select a blade tip (r/R=0.95)pitch that is 10 percent greater than the constant pitch propeller. 3)select a blade section pitch at r/R=0.7 equal to the pitch of theoptimum constant pitch propeller.

When a curve is fitted to these three points on a graph of r/R versuspitch, a non-linear pitch distribution is shown. The resulting modifiedpropeller will be identical to the optimum constant pitch propeller inall geometry details except for the non-linear pitch distribution.

In the case of the test craft, a 26 inch diameter by 28 inch constantpitch Columbian Bronze Corp. "Tetradyne" propeller was found to beoptimum using standard empirical charts. A second 26 inch diameter"Tetradyne" propeller was purchased with a 25 inch constant pitch. Thissecond propeller was then mechanically repitched to 28 inches at r/R=0.7and to 31 inches at r/R=0.95. This is the propeller which was matched tothe asymmetric pre-swirl vanes, and which also performed exceptionallywell with no vanes ahead of it.

Since air and water are both fluids and the present invention is anapplication of fluid mechanics, it is obvious that the principles of thepresent invention can be applied to an airplane propeller to improve itsoperating efficiency when the propeller is not absolutely vertical. Forexample, a propeller on a pusher type configuration could probablybenefit from the application of the principles of the present inventionto its operation. As shown in FIG. 13, vanes 40 of the present inventionwould be mounted on the exterior of engine casing 42 on the side wherepropeller 44 is on the upward part of its rotation, thereby givingpropeller 44 a positive angle of atack on that side of its disc. As witha boat, the size and angular orientation of the vanes for an airplanepropeller will be dictated by the flow at the propeller.

What is claimed is:
 1. An improved propeller for a marine vessel, the blades of said propeller being designed by calculating the pitch for a constant pitch propeller for that vessel and then decreasing the pitch at the roots of the blades by approximately 10% and increasing the pitch at the tips by approximately 10%, with the change in pitch being a smooth curve such that the pitch at approximately 70% radius remains unchanged from that of the constant pitch propeller.
 2. A propeller as in claim 1 wherein said propeller is mounted on an inclined shaft.
 3. A propeller as in claim 2 further including means for directing the flow of water into said propeller to offset the effects of the inclination of the shaft.
 4. An improved propeller for a marine vessel, the blades of said propeller having a pitch at their roots that is approximately 10% less than that calculated for a constant pitch propeller for the same vessel and a pitch at their tips that is approximately 10% greater than that calculated for a constant pitch propeller for the same vessel, with the change in pitch of said propeller being a smooth curve such that the pitch at approximately 70% radius is unchanged from that of a constant pitch propeller for the same vessel.
 5. A propeller as in claim 4 wherein said propeller is mounted on an inclined shaft.
 6. A propeller as in claim 5 further including means for directing the flow of water into said propeller to offset the effects of the inclination of the shaft.
 7. The method of designing a propeller for a marine vessel which comprises calculating the pitch for the blades of a constant pitch propeller for that vessel, and then modifying the pitch as follows: decreasing the pitch at the roots by approximately 10% and increasing the pitch at the tips by approximately 10%, with the change in pitch being a smooth curve such that the pitch at approximately 70% radius is unchanged.
 8. A propeller made according to the method of claim
 7. 